I am fairly confident that this is a Slow Worm, Anguis fragilis, so more of a lizard than a true snake - but a reptile nonetheless. From what I can see from the image at higher magnification what appears to be a mouth is in fact an opening into the body cavity where the head once was. Slow Worms are vulnerable to predation from many larger birds, including hawks, owls and crows etc. and predatory mammals. That said if it is the remains of a meal the perpetrator was obviously not very hungry.